Are Dihydroxyphenylalanine Decarboxylase and 5‐Hydroxytryptophan Decarboxylase Individual Enzymes?
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Abstract Using an enzyme preparation from rabbit kidney cortex, the amino acids 3,4‐dihydroxyphenylalanine and 5‐hydroxytryptophan were found to inhibit each other's enzymatic decarboxylation competitively. Competitive inhibitions of these reactions by m‐tyrosine, o‐tyrosine, and caffeic acid were also observed. After purification of an enzyme preparation on a column of DEAE cellulose a peak appeared which contained most of the dihydroxyphenylalanine decarboxylase activity present in the extract. In this peak also nearly all the 5‐hydroxytryptophan decarboxylase activity was found. The ratio of the two activities was practically the same throughout the peak. The data were in agreement with the assumption that dihydroxyphenylalanine and 5‐hydroxytryptophan were decarboxylated by one enzyme.Keywords:
Dihydroxyphenylalanine
Decarboxylation
5-Hydroxytryptophan
Decarboxylation
Dihydroxyphenylalanine
5-Hydroxytryptophan
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Abstract The labeled 5‐hydroxytryptophan (I) is prepared and then incubated with mammalian aromatic L‐amino acid decarboxylase from hog kidney.
Decarboxylation
5-Hydroxytryptophan
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Decarboxylation
Dihydroxyphenylalanine
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Abstract —DOPA and 5‐hydroxytryptophan (5‐HTP) are generally supposed to be decarboxylated in mammalian tissues by a single enzyme, the two activities being present in constant ratio through a variety of purification procedures. It has now been shown that the ratio of activity of the liver enzyme towards the two substrates can be altered by mild treatments, such as might be used in solubilization of brain preparations. DOPA decarboxylase activity was preferentially inactivated by sodium dodecyl sulphate treatment, and 5‐HTP decarboxylation by urea. Previous reports that the two substrates show different pH optima but are mutually competitive, have been confirmed. The K m of the enzyme towards 5‐HTP was lowest at pH 7.8 (the optimum pH for decarboxylation of this amino acid), but the variation with pH of the K m towards DOPA was unrelated to the pH optimum for decarboxylation. There appeared to be no relation between the probable ionization state of the substrates and the pH dependence of the enzyme. Studies on the binding characteristics of the enzyme for the two products, dopamine and serotonin, did not show any specific saturable binding. It is proposed that the enzyme has a complex active site, with separate affinity sites for the two substrates, adjacent to a single catalytic site.
Decarboxylation
5-Hydroxytryptophan
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Decarboxylation
Dihydroxyphenylalanine
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Amino acid decarboxylation activity in dispersed rat pancreas acinar cells and fractions derived by differential centrifugation of homogenate of these cells was studied. The rate of decarboxylation was measured by determining the rate of production of the [3H]-amine from [3H]-amino acid or the rate of production of 14C(>2 from the [14C]-carboxy-labelled amino acid. Only the hydroxylated amino acids L-dopa and 5-hydroxytryptophan are decarboxylated by intact dispersed pancreas acinar cells or cell homogenates at all pH values and amino acid concentrations tested. The decarboxylase activity is located exclusively in the cell cytosol. Each substrate competitively inhibits the decarboxylation of the other and the decarboxylation of each is inhibited by NSD-1055. The estimated Km and Vmax are, for L-dopa, 4.8 × 10–5M and 2.5 nmol/mg protein/min and for 5-hydroxytryptophan, 2.9 × 10–5M and 0.3 nmol/mg protein/min. The pH optimum for 5-hydroxytryptophan decarboxylation is from 7.0–8.5 while that for L-dopa is 7.0. It is concluded that pancreas acinar cells possess a single aromatic amino acid decarboxylase specific for the hydroxylated amino acids L-dopa and 5-hydroxytryptophan, and which is similar in all properties studied to the aromatic amino acid decarboxylase found in several other mammalian tissues.
Decarboxylation
5-Hydroxytryptophan
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Decarboxylation
Dihydroxyphenylalanine
Pyridoxal phosphate
Pyridoxal 5-Phosphate
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5-Hydroxytryptophan
Dihydroxyphenylalanine
Phenylalanine hydroxylase
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Abstract Using an enzyme preparation from rabbit kidney cortex, the amino acids 3,4‐dihydroxyphenylalanine and 5‐hydroxytryptophan were found to inhibit each other's enzymatic decarboxylation competitively. Competitive inhibitions of these reactions by m‐tyrosine, o‐tyrosine, and caffeic acid were also observed. After purification of an enzyme preparation on a column of DEAE cellulose a peak appeared which contained most of the dihydroxyphenylalanine decarboxylase activity present in the extract. In this peak also nearly all the 5‐hydroxytryptophan decarboxylase activity was found. The ratio of the two activities was practically the same throughout the peak. The data were in agreement with the assumption that dihydroxyphenylalanine and 5‐hydroxytryptophan were decarboxylated by one enzyme.
Dihydroxyphenylalanine
Decarboxylation
5-Hydroxytryptophan
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5-Hydroxytryptophan
Dihydroxyphenylalanine
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